1.
Biosignal and
Biomedical Image
Processing
MATLAB-Based Applications
JOHN L. SEMMLOW
Robert WoodJohnson Medical School
New Brunswick, New Jersey, U.S.A.
Rutgers University
Piscataway, New Jersey, U.S.A.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
2.
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3.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
4.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
5.
To Lawrence Stark, M.D., who has shown me the many possibilities . . .
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
6.
Series Introduction
Over the past 50 years, digital signal processing has evolved as a major engi-
neering discipline. The fields of signal processing have grown from the origin
of fast Fourier transform and digital filter design to statistical spectral analysis
and array processing, image, audio, and multimedia processing, and shaped de-
velopments in high-performance VLSI signal processor design. Indeed, there
are few fields that enjoy so many applications—signal processing is everywhere
in our lives.
When one uses a cellular phone, the voice is compressed, coded, and
modulated using signal processing techniques. As a cruise missile winds along
hillsides searching for the target, the signal processor is busy processing the
images taken along the way. When we are watching a movie in HDTV, millions
of audio and video data are being sent to our homes and received with unbeliev-
able fidelity. When scientists compare DNA samples, fast pattern recognition
techniques are being used. On and on, one can see the impact of signal process-
ing in almost every engineering and scientific discipline.
Because of the immense importance of signal processing and the fast-
growing demands of business and industry, this series on signal processing
serves to report up-to-date developments and advances in the field. The topics
of interest include but are not limited to the following:
• Signal theory and analysis
• Statistical signal processing
• Speech and audio processing
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
7.
• Image and video processing
• Multimedia signal processing and technology
• Signal processing for communications
• Signal processing architectures and VLSI design
We hope this series will provide the interested audience with high-quality,
state-of-the-art signal processing literature through research monographs, edited
books, and rigorously written textbooks by experts in their fields.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
8.
Preface
Signal processing can be broadly defined as the application of analog or digital
techniques to improve the utility of a data stream. In biomedical engineering
applications, improved utility usually means the data provide better diagnostic
information. Analog techniques are applied to a data stream embodied as a time-
varying electrical signal while in the digital domain the data are represented as
an array of numbers. This array could be the digital representation of a time-
varying signal, or an image. This text deals exclusively with signal processing
of digital data, although Chapter 1 briefly describes analog processes commonly
found in medical devices.
This text should be of interest to a broad spectrum of engineers, but it
is written specifically for biomedical engineers (also known as bioengineers).
Although the applications are different, the signal processing methodology used
by biomedical engineers is identical to that used by other engineers such electri-
cal and communications engineers. The major difference for biomedical engi-
neers is in the level of understanding required for appropriate use of this technol-
ogy. An electrical engineer may be required to expand or modify signal
processing tools, while for biomedical engineers, signal processing techniques
are tools to be used. For the biomedical engineer, a detailed understanding of
the underlying theory, while always of value, may not be essential. Moreover,
considering the broad range of knowledge required to be effective in this field,
encompassing both medical and engineering domains, an in-depth understanding
of all of the useful technology is not realistic. It is important is to know what
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
9.
tools are available, have a good understanding of what they do (if not how they
do it), be aware of the most likely pitfalls and misapplications, and know how
to implement these tools given available software packages. The basic concept
of this text is that, just as the cardiologist can benefit from an oscilloscope-type
display of the ECG without a deep understanding of electronics, so a biomedical
engineer can benefit from advanced signal processing tools without always un-
derstanding the details of the underlying mathematics.
As a reflection of this philosophy, most of the concepts covered in this
text are presented in two sections. The first part provides a broad, general under-
standing of the approach sufficient to allow intelligent application of the con-
cepts. The second part describes how these tools can be implemented and relies
primarily on the MATLAB software package and several of its toolboxes.
This text is written for a single-semester course combining signal and
image processing. Classroom experience using notes from this text indicates
that this ambitious objective is possible for most graduate formats, although
eliminating a few topics may be desirable. For example, some of the introduc-
tory or basic material covered in Chapters 1 and 2 could be skipped or treated
lightly for students with the appropriate prerequisites. In addition, topics such
as advanced spectral methods (Chapter 5), time-frequency analysis (Chapter 6),
wavelets (Chapter 7), advanced filters (Chapter 8), and multivariate analysis
(Chapter 9) are pedagogically independent and can be covered as desired with-
out affecting the other material.
Although much of the material covered here will be new to most students,
the book is not intended as an “introductory” text since the goal is to provide a
working knowledge of the topics presented without the need for additional
course work. The challenge of covering a broad range of topics at a useful,
working depth is motivated by current trends in biomedical engineering educa-
tion, particularly at the graduate level where a comprehensive education must
be attained with a minimum number of courses. This has led to the development
of “core” courses to be taken by all students. This text was written for just such
a core course in the Graduate Program of Biomedical Engineering at Rutgers
University. It is also quite suitable for an upper-level undergraduate course and
would be of value for students in other disciplines who would benefit from a
working knowledge of signal and image processing.
It would not be possible to cover such a broad spectrum of material to a
depth that enables productive application without heavy reliance on MATLAB-
based examples and problems. In this regard, the text assumes the student
has some knowledge of MATLAB programming and has available the basic
MATLAB software package including the Signal Processing and Image Process-
ing Toolboxes. (MATLAB also produces a Wavelet Toolbox, but the section on
wavelets is written so as not to require this toolbox, primarily to keep the num-
ber of required toolboxes to a minimum.) The problems are an essential part of
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
10.
this text and often provide a discovery-like experience regarding the associated
topic. A few peripheral topics are introduced only though the problems. The
code used for all examples is provided in the CD accompanying this text. Since
many of the problems are extensions or modifications of examples given in the
chapter, some of the coding time can be reduced by starting with the code of a
related example. The CD also includes support routines and data files used in
the examples and problems. Finally, the CD contains the code used to generate
many of the figures. For instructors, there is a CD available that contains the
problem solutions and Powerpoint presentations from each of the chapters.
These presentations include figures, equations, and text slides related to chapter.
Presentations can be modified by the instructor as desired.
In addition to heavy reliance on MATLAB problems and examples, this
text makes extensive use of simulated data. Except for the section on image
processing, examples involving biological signals are rarely used. In my view,
examples using biological signals provide motivation, but they are not generally
very instructive. Given the wide range of material to be presented at a working
depth, emphasis is placed on learning the tools of signal processing; motivation
is left to the reader (or the instructor).
Organization of the text is straightforward. Chapters 1 through 4 are fairly
basic. Chapter 1 covers topics related to analog signal processing and data acqui-
sition while Chapter 2 includes topics that are basic to all aspects of signal and
image processing. Chapters 3 and 4 cover classical spectral analysis and basic
digital filtering, topics fundamental to any signal processing course. Advanced
spectral methods, covered in Chapter 5, are important due to their widespread
use in biomedical engineering. Chapter 6 and the first part of Chapter 7 cover
topics related to spectral analysis when the signal’s spectrum is varying in time,
a condition often found in biological signals. Chapter 7 also covers both contin-
uous and discrete wavelets, another popular technique used in the analysis of
biomedical signals. Chapters 8 and 9 feature advanced topics. In Chapter 8,
optimal and adaptive filters are covered, the latter’s inclusion is also motivated
by the time-varying nature of many biological signals. Chapter 9 introduces
multivariate techniques, specifically principal component analysis and indepen-
dent component analysis, two analysis approaches that are experiencing rapid
growth with regard to biomedical applications. The last four chapters cover
image processing, with the first of these, Chapter 10, covering the conventions
used by MATLAB’s Imaging Processing Toolbox. Image processing is a vast
area and the material covered here is limited primarily to areas associated with
medical imaging: image acquisition (Chapter 13); image filtering, enhancement,
and transformation (Chapter 11); and segmentation, and registration (Chapter 12).
Many of the chapters cover topics that can be adequately covered only in
a book dedicated solely to these topics. In this sense, every chapter represents
a serious compromise with respect to comprehensive coverage of the associated
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
11.
topics. My only excuse for any omissions is that classroom experience with this
approach seems to work: students end up with a working knowledge of a vast
array of signal and image processing tools. A few of the classic or major books
on these topics are cited in an Annotated bibliography at the end of the book.
No effort has been made to construct an extensive bibliography or reference list
since more current lists would be readily available on the Web.
TEXTBOOK PROTOCOLS
In most early examples that feature MATLAB code, the code is presented in
full, while in the later examples some of the routine code (such as for plotting,
display, and labeling operation) is omitted. Nevertheless, I recommend that stu-
dents carefully label (and scale when appropriate) all graphs done in the prob-
lems. Some effort has been made to use consistent notation as described in
Table 1. In general, lower-case letters n and k are used as data subscripts, and
capital letters, N and K are used to indicate the length (or maximum subscript
value) of a data set. In two-dimensional data sets, lower-case letters m and n
are used to indicate the row and column subscripts of an array, while capital
letters M and N are used to indicate vertical and horizontal dimensions, respec-
tively. The letter m is also used as the index of a variable produced by a transfor-
mation, or as an index indicating a particular member of a family of related
functions.* While it is common to use brackets to enclose subscripts of discrete
variables (i.e., x[n]), ordinary parentheses are used here. Brackets are reserved
to indicate vectors (i.e., [x1, x2, x3 , . . . ]) following MATLAB convention.
Other notation follows standard conventions.
Italics (“) are used to introduce important new terms that should be incor-
porated into the reader’s vocabulary. If the meaning of these terms is not obvi-
ous from their use, they are explained where they are introduced. All MATLAB
commands, routines, variables, and code are shown in the Courier typeface.
Single quotes are used to highlight MATLAB filenames or string variables.
Textbook protocols are summarized in Table 1.
I wish to thank Susanne Oldham who managed to edit this book, and
provided strong, continuing encouragement and support. I would also like to
acknowledge the patience and support of Peggy Christ and Lynn Hutchings.
Professor Shankar Muthu Krishnan of Singapore provided a very thoughtful
critique of the manuscript which led to significant improvements. Finally, I
thank my students who provided suggestions and whose enthusiasm for the
material provided much needed motivation.
*For example, m would be used to indicate the harmonic number of a family of harmonically related
sine functions; i.e., fm(t) = sin (2 π m t).
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
12.
TABLE 1 Textbook Conventions
Symbol Description/General usage
x(t), y(t) General functions of time, usually a waveform or signal
k, n Data indices, particularly for digitized time data
K, N Maximum index or size of a data set
x(n), y(n) Waveform variable, usually digitized time variables (i.e., a dis-
creet variable)
m Index of variable produced by transformation, or the index of
specifying the member number of a family of functions (i.e.,
fm(t))
X(f), Y(f) Frequency representation (complex) of a time function
X(m), Y(m) Frequency representation (complex) of a discreet variable
h(t) Impulse response of a linear system
h(n) Discrete impulse response of a linear system
b(n) Digital filter coefficients representing the numerator of the dis-
creet Transfer Function; hence the same as the impulse re-
sponse
a(n) Digital filter coefficients representing the denominator of the dis-
creet Transfer Function
Courier font MATLAB command, variable, routine, or program.
Courier font MATLAB filename or string variable
John L. Semmlow
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
13.
Contents
Preface
1 Introduction
Typical Measurement Systems
Transducers
Further Study: The Transducer
Analog Signal Processing
Sources of Variability: Noise
Electronic Noise
Signal-to-Noise Ratio
Analog Filters: Filter Basics
Filter Types
Filter Bandwidth
Filter Order
Filter Initial Sharpness
Analog-to-Digital Conversion: Basic Concepts
Analog-to-Digital Conversion Techniques
Quantization Error
Further Study: Successive Approximation
Time Sampling: Basics
Further Study: Buffering and Real-Time Data Processing
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
14.
Data Banks
Problems
2 Basic Concepts
Noise
Ensemble Averaging
MATLAB Implementation
Data Functions and Transforms
Convolution, Correlation, and Covariance
Convolution and the Impulse Response
Covariance and Correlation
MATLAB Implementation
Sampling Theory and Finite Data Considerations
Edge Effects
Problems
3 Spectral Analysis: Classical Methods
Introduction
The Fourier Transform: Fourier Series Analysis
Periodic Functions
Symmetry
Discrete Time Fourier Analysis
Aperiodic Functions
Frequency Resolution
Truncated Fourier Analysis: Data Windowing
Power Spectrum
MATLAB Implementation
Direct FFT and Windowing
The Welch Method for Power Spectral Density Determination
Widow Functions
Problems
4 Digital Filters
The Z-Transform
Digital Transfer Function
MATLAB Implementation
Finite Impulse Response (FIR) Filters
FIR Filter Design
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
15.
Derivative Operation: The Two-Point Central Difference
Algorithm
MATLAB Implementation
Infinite Impulse Response (IIR) Filters
Filter Design and Application Using the MATLAB Signal
Processing Toolbox
FIR Filters
Two-Stage FIR Filter Design
Three-Stage Filter Design
IIR Filters
Two-Stage IIR Filter Design
Three-Stage IIR Filter Design: Analog Style Filters
Problems
5 Spectral Analysis: Modern Techniques
Parametric Model-Based Methods
MATLAB Implementation
Non-Parametric Eigenanalysis Frequency Estimation
MATLAB Implementation
Problems
6 Time–Frequency Methods
Basic Approaches
Short-Term Fourier Transform: The Spectrogram
Wigner-Ville Distribution: A Special Case of Cohen’s Class
Choi-Williams and Other Distributions
Analytic Signal
MATLAB Implementation
The Short-Term Fourier Transform
Wigner-Ville Distribution
Choi-Williams and Other Distributions
Problems
7 The Wavelet Transform
Introduction
The Continuous Wavelet Transform
Wavelet Time—Frequency Characteristics
MATLAB Implementation
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
16.
The Discrete Wavelet Transform
Filter Banks
The Relationship Between Analytical Expressions and
Filter Banks
MATLAB Implementation
Denoising
Discontinuity Detection
Feature Detection: Wavelet Packets
Problems
8 Advanced Signal Processing Techniques:
Optimal and Adaptive Filters
Optimal Signal Processing: Wiener Filters
MATLAB Implementation
Adaptive Signal Processing
Adaptive Noise Cancellation
MATLAB Implementation
Phase Sensitive Detection
AM Modulation
Phase Sensitive Detectors
MATLAB Implementation
Problems
9 Multivariate Analyses: Principal Component Analysis
and Independent Component Analysis
Introduction
Principal Component Analysis
Order Selection
MATLAB Implementation
Data Rotation
Principal Component Analysis Evaluation
Independent Component Analysis
MATLAB Implementation
Problems
10 Fundamentals of Image Processing: MATLAB Image
Processing Toolbox
Image Processing Basics: MATLAB Image Formats
General Image Formats: Image Array Indexing
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
17.
Data Classes: Intensity Coding Schemes
Data Formats
Data Conversions
Image Display
Image Storage and Retrieval
Basic Arithmetic Operations
Advanced Protocols: Block Processing
Sliding Neighborhood Operations
Distinct Block Operations
Problems
11 Image Processing: Filters, Transformations,
and Registration
Spectral Analysis: The Fourier Transform
MATLAB Implementation
Linear Filtering
MATLAB Implementation
Filter Design
Spatial Transformations
MATLAB Implementation
Affine Transformations
General Affine Transformations
Projective Transformations
Image Registration
Unaided Image Registration
Interactive Image Registration
Problems
12 Image Segmentation
Pixel-Based Methods
Threshold Level Adjustment
MATLAB Implementation
Continuity-Based Methods
MATLAB Implementation
Multi-Thresholding
Morphological Operations
MATLAB Implementation
Edge-Based Segmentation
MATLAB Implementation
Problems
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
18.
13 Image Reconstruction
CT, PET, and SPECT
Fan Beam Geometry
MATLAB Implementation
Radon Transform
Inverse Radon Transform: Parallel Beam Geometry
Radon and Inverse Radon Transform: Fan Beam Geometry
Magnetic Resonance Imaging
Basic Principles
Data Acquisition: Pulse Sequences
Functional MRI
MATLAB Implementation
Principal Component and Independent Component Analysis
Problems
Annotated Bibliography
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
19.
Annotated Bibliography
The following is a very selective list of books or articles that will be of value of in
providing greater depth and mathematical rigor to the material presented in this text.
Comments regarding the particular strengths of the reference are included.
Akansu, A. N. and Haddad, R. A., Multiresolution Signal Decomposition: Transforms,
subbands, wavelets. Academic Press, San Diego CA, 1992. A modern classic that
presents, among other things, some of the underlying theoretical aspects of wavelet
analysis.
Aldroubi A and Unser, M. (eds) Wavelets in Medicine and Biology, CRC Press, Boca
Raton, FL, 1996. Presents a variety of applications of wavelet analysis to biomedical
engineering.
Boashash, B. Time-Frequency Signal Analysis, Longman Cheshire Pty Ltd., 1992. Early
chapters provide a very useful introduction to time–frequency analysis followed by a
number of medical applications.
Boashash, B. and Black, P.J. An efficient real-time implementation of the Wigner-Ville
Distribution, IEEE Trans. Acoust. Speech Sig. Proc. ASSP-35:1611–1618, 1987.
Practical information on calculating the Wigner-Ville distribution.
Boudreaux-Bartels, G. F. and Murry, R. Time-frequency signal representations for bio-
medical signals. In: The Biomedical Engineering Handbook. J. Bronzino (ed.) CRC
Press, Boca Raton, Florida and IEEE Press, Piscataway, N.J., 1995. This article pres-
ents an exhaustive, or very nearly so, compilation of Cohen’s class of time-frequency
distributions.
Bruce, E. N. Biomedical Signal Processing and Signal Modeling, John Wiley and Sons,
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
20.
New York, 2001. Rigorous treatment with more of an emphasis on linear systems
than signal processing. Introduces nonlinear concepts such as chaos.
Cichicki, A and Amari S. Adaptive Bilnd Signal and Image Processing: Learning Algo-
rithms and Applications, John Wiley and Sons, Inc. New York, 2002. Rigorous,
somewhat dense, treatment of a wide range of principal component and independent
component approaches. Includes disk.
Cohen, L. Time-frequency distributions—A review. Proc. IEEE 77:941–981, 1989.
Classic review article on the various time-frequency methods in Cohen’s class of
time–frequency distributions.
Ferrara, E. and Widrow, B. Fetal Electrocardiogram enhancement by time-sequenced
adaptive filtering. IEEE Trans. Biomed. Engr. BME-29:458–459, 1982. Early appli-
cation of adaptive noise cancellation to a biomedical engineering problem by one of
the founders of the field. See also Widrow below.
Friston, K. Statistical Parametric Mapping On-line at: http://www.fil.ion.ucl.ac.uk/spm/
course/note02/ Through discussion of practical aspects of fMRI analysis including
pre-processing, statistical methods, and experimental design. Based around SPM anal-
ysis software capabilities.
Haykin, S. Adaptive Filter Theory (2nd
ed.), Prentice-Hall, Inc., Englewood Cliffs, N.J.,
1991. The definitive text on adaptive filters including Weiner filters and gradient-
based algorithms.
Hyva¨rinen, A. Karhunen, J. and Oja, E. Independent Component Analysis, John Wiley
and Sons, Inc. New York, 2001. Fundamental, comprehensive, yet readable book on
independent component analysis. Also provides a good review of principal compo-
nent analysis.
Hubbard B.B. The World According to Wavelets (2nd
ed.) A.K. Peters, Ltd. Natick, MA,
1998. Very readable introductory book on wavelengths including an excellent section
on the foyer transformed. Can be read by a non-signal processing friend.
Ingle, V.K. and Proakis, J. G. Digital Signal Processing with MATLAB, Brooks/Cole,
Inc. Pacific Grove, CA, 2000. Excellent treatment of classical signal processing meth-
ods including the Fourier transform and both FIR and IIR digital filters. Brief, but
informative section on adaptive filtering.
Jackson, J. E. A User’s Guide to Principal Components, John Wiley and Sons, New
York, 1991. Classic book providing everything you ever want to know about principal
component analysis. Also covers linear modeling and introduces factor analysis.
Johnson, D.D. Applied Multivariate Methods for Data Analysis, Brooks/Cole, Pacific
Grove, CA, 1988. Careful, detailed coverage of multivariate methods including prin-
cipal components analysis. Good coverage of discriminant analysis techniques.
Kak, A.C and Slaney M. Principles of Computerized Tomographic Imaging. IEEE Press,
New York, 1988. Thorough, understandable treatment of algorithms for reconstruc-
tion of tomographic images including both parallel and fan-beam geometry. Also
includes techniques used in reflection tomography as occurs in ultrasound imaging.
Marple, S.L. Digital Spectral Analysis with Applications, Prentice-Hall, Englewood
Cliffs, NJ, 1987. Classic text on modern spectral analysis methods. In-depth, rigorous
treatment of Fourier transform, parametric modeling methods (including AR and
ARMA), and eigenanalysis-based techniques.
Rao, R.M. and Bopardikar, A.S. Wavelet Transforms: Introduction to Theory and Appli-
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
21.
cations, Addison-Wesley, Inc., Reading, MA, 1998. Good development of wavelet
analysis including both the continuous and discreet wavelet transforms.
Shiavi, R Introduction to Applied Statistical Signal Analysis, (2nd
ed), Academic Press,
San Diego, CA, 1999. Emphasizes spectral analysis of signals buried in noise. Excel-
lent coverage of Fourier analysis, and autoregressive methods. Good introduction to
statistical signal processing concepts.
Sonka, M., Hlavac V., and Boyle R. Image processing, analysis, and machine vision.
Chapman and Hall Computing, London, 1993. A good description of edge-based and
other segmentation methods.
Strang, G and Nguyen, T. Wavelets and Filter Banks, Wellesley-Cambridge Press,
Wellesley, MA, 1997. Thorough coverage of wavelet filter banks including extensive
mathematical background.
Stearns, S.D. and David, R.A Signal Processing Algorithms in MATLAB, Prentice Hall,
Upper Saddle River, NJ, 1996. Good treatment of the classical Fourier transform and
digital filters. Also covers the LMS adaptive filter algorithm. Disk enclosed.
Wickerhauser, M.V. Adapted Wavelet Analysis from Theory to Software, A.K. Peters,
Ltd. and IEEE Press, Wellesley, MA, 1994. Rigorous, extensive treatment of wavelet
analysis.
Widrow, B. Adaptive noise cancelling: Principles and applications. Proc IEEE 63:1692–
1716, 1975. Classic original article on adaptive noise cancellation.
Wright S. Nuclear Magnetic Resonance and Magnetic Resonance Imaging. In: Introduc-
tion to Biomedical Engineering (Enderle, Blanchard and Bronzino, Eds.) Academic
Press, San Diego, CA, 2000. Good mathematical development of the physics of MRI
using classical concepts.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
22.
1
Introduction
TYPICAL MEASUREMENT SYSTEMS
A schematic representation of a typical biomedical measurement system is
shown in Figure 1.1. Here we use the term measurement in the most general
sense to include image acquisition or the acquisition of other forms of diagnostic
information. The physiological process of interest is converted into an electric
FIGURE 1.1 Schematic representation of typical bioengineering measurement
system.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
23.
signal via the transducer (Figure 1.1). Some analog signal processing is usually
required, often including amplification and lowpass (or bandpass) filtering.
Since most signal processing is easier to implement using digital methods, the
analog signal is converted to digital format using an analog-to-digital converter.
Once converted, the signal is often stored, or buffered, in memory to facilitate
subsequent signal processing. Alternatively, in some real-time* applications, the
incoming data must be processed as quickly as possible with minimal buffering,
and may not need to be permanently stored. Digital signal processing algorithms
can then be applied to the digitized signal. These signal processing techniques
can take a wide variety of forms and various levels of sophistication, and they
make up the major topic area of this book. Some sort of output is necessary in
any useful system. This usually takes the form of a display, as in imaging sys-
tems, but may be some type of an effector mechanism such as in an automated
drug delivery system.
With the exception of this chapter, this book is limited to digital signal
and image processing concerns. To the extent possible, each topic is introduced
with the minimum amount of information required to use and understand the
approach, and enough information to apply the methodology in an intelligent
manner. Understanding of strengths and weaknesses of the various methods is
also covered, particularly through discovery in the problems at the end of the
chapter. Hence, the problems at the end of each chapter, most of which utilize
the MATLABTM
software package (Waltham, MA), constitute an integral part
of the book: a few topics are introduced only in the problems.
A fundamental assumption of this text is that an in-depth mathematical
treatment of signal processing methodology is not essential for effective and
appropriate application of these tools. Thus, this text is designed to develop
skills in the application of signal and image processing technology, but may not
provide the skills necessary to develop new techniques and algorithms. Refer-
ences are provided for those who need to move beyond application of signal
and image processing tools to the design and development of new methodology.
In subsequent chapters, each major section is followed by a section on imple-
mentation using the MATLAB software package. Fluency with the MATLAB
language is assumed and is essential for the use of this text. Where appropriate,
a topic area may also include a more in-depth treatment including some of the
underlying mathematics.
*Learning the vocabulary is an important part of mastering a discipline. In this text we highlight,
using italics, terms commonly used in signal and image processing. Sometimes the highlighted term
is described when it is introduced, but occasionally determination of its definition is left to responsi-
bility of the reader. Real-time processing and buffering are described in the section on analog-to-
digital conversion.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
24.
TRANSDUCERS
A transducer is a device that converts energy from one form to another. By this
definition, a light bulb or a motor is a transducer. In signal processing applica-
tions, the purpose of energy conversion is to transfer information, not to trans-
form energy as with a light bulb or a motor. In measurement systems, all trans-
ducers are so-called input transducers, they convert non-electrical energy into
an electronic signal. An exception to this is the electrode, a transducer that
converts electrical energy from ionic to electronic form. Usually, the output of
a biomedical transducer is a voltage (or current) whose amplitude is proportional
to the measured energy.
The energy that is converted by the input transducer may be generated by
the physiological process itself, indirectly related to the physiological process,
or produced by an external source. In the last case, the externally generated
energy interacts with, and is modified by, the physiological process, and it is
this alteration that produces the measurement. For example, when externally
produced x-rays are transmitted through the body, they are absorbed by the
intervening tissue, and a measurement of this absorption is used to construct an
image. Many diagnostically useful imaging systems are based on this external
energy approach.
In addition to passing external energy through the body, some images are
generated using the energy of radioactive emissions of radioisotopes injected
into the body. These techniques make use of the fact that selected, or tagged,
molecules will collect in specific tissue. The areas where these radioisotopes
collect can be mapped using a gamma camera, or with certain short-lived iso-
topes, better localized using positron emission tomography (PET).
Many physiological processes produce energy that can be detected di-
rectly. For example, cardiac internal pressures are usually measured using a
pressure transducer placed on the tip of catheter introduced into the appropriate
chamber of the heart. The measurement of electrical activity in the heart, mus-
cles, or brain provides other examples of the direct measurement of physiologi-
cal energy. For these measurements, the energy is already electrical and only
needs to be converted from ionic to electronic current using an electrode. These
sources are usually given the term ExG, where the ‘x’ represents the physiologi-
cal process that produces the electrical energy: ECG–electrocardiogram, EEG–
electroencephalogram; EMG–electromyogram; EOG–electrooculargram, ERG–
electroretiniogram; and EGG–electrogastrogram. An exception to this terminology
is the electrical activity generated by this skin which is termed the galvanic skin
response, GSR. Typical physiological energies and the applications that use
these energy forms are shown in Table 1.1
The biotransducer is often the most critical element in the system since it
constitutes the interface between the subject or life process and the rest of the
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
25.
TABLE 1.1 Energy Forms and Related Direct Measurements
Energy Measurement
Mechanical
length, position, and velocity muscle movement, cardiovascular pressures,
muscle contractility
force and pressure valve and other cardiac sounds
Heat body temperature, thermography
Electrical EEG, ECG, EMG, EOG, ERG, EGG, GSR
Chemical ion concentrations
system. The transducer establishes the risk, or noninvasiveness, of the overall
system. For example, an imaging system based on differential absorption of
x-rays, such as a CT (computed tomography) scanner is considered more inva-
sive than an imagining system based on ultrasonic reflection since CT uses
ionizing radiation that may have an associated risk. (The actual risk of ionizing
radiation is still an open question and imaging systems based on x-ray absorp-
tion are considered minimally invasive.) Both ultrasound and x-ray imaging
would be considered less invasive than, for example, monitoring internal cardiac
pressures through cardiac catherization in which a small catheter is treaded into
the heart chambers. Indeed many of the outstanding problems in biomedical
measurement, such as noninvasive measurement of internal cardiac pressures,
or the noninvasive measurement of intracranial pressure, await an appropriate
(and undoubtedly clever) transducer mechanism.
Further Study: The Transducer
The transducer often establishes the major performance criterion of the system.
In a later section, we list and define a number of criteria that apply to measure-
ment systems; however, in practice, measurement resolution, and to a lesser
extent bandwidth, are generally the two most important and troublesome mea-
surement criteria. In fact, it is usually possible to trade-off between these two
criteria. Both of these criteria are usually established by the transducer. Hence,
although it is not the topic of this text, good system design usually calls for care
in the choice or design of the transducer element(s). An efficient, low-noise
transducer design can often reduce the need for extensive subsequent signal
processing and still produce a better measurement.
Input transducers use one of two different fundamental approaches: the
input energy causes the transducer element to generate a voltage or current, or
the input energy creates a change in the electrical properties (i.e., the resistance,
inductance, or capacitance) of the transducer element. Most optical transducers
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
26.
use the first approach. Photons strike a photo sensitive material producing free
electrons (or holes) that can then be detected as an external current flow. Piezo-
electric devices used in ultrasound also generate a charge when under mechani-
cal stress. Many examples can be found of the use of the second category, a
change in some electrical property. For example, metals (and semiconductors)
undergo a consistent change in resistance with changes in temperature, and most
temperature transducers utilize this feature. Other examples include the strain
gage, which measures mechanical deformation using the small change in resis-
tance that occurs when the sensing material is stretched.
Many critical problems in medical diagnosis await the development of
new approaches and new transducers. For example, coronary artery disease is a
major cause of death in developed countries, and its treatment would greatly
benefit from early detection. To facilitate early detection, a biomedical instru-
mentation system is required that is inexpensive and easy to operate so that it
could be used for general screening. In coronary artery disease, blood flow to
the arteries of the heart (i.e., coronaries) is reduced due to partial or complete
blockage (i.e., stenoses). One conceptually simple and inexpensive approach is
to detect the sounds generated by turbulent blood flow through partially in-
cluded coronary arteries (called bruits when detected in other arteries such as
the carotids). This approach requires a highly sensitive transducer(s), in this case
a cardiac microphone, as well as advanced signal processing methods. Results of
efforts based on this approach are ongoing, and the problem of noninvasive
detection of coronary artery disease is not yet fully solved.
Other holy grails of diagnostic cardiology include noninvasive measure-
ment of cardiac output (i.e., volume of blood flow pumped by the heart per unit
time) and noninvasive measurement of internal cardiac pressures. The former
has been approached using Doppler ultrasound, but this technique has not yet
been accepted as reliable. Financial gain and modest fame awaits the biomedical
engineer who develops instrumentation that adequately addresses any of these
three outstanding measurement problems.
ANALOG SIGNAL PROCESSING
While the most extensive signal processing is usually performed on digitized
data using algorithms implemented in software, some analog signal processing
is usually necessary. The first analog stage depends on the basic transducer
operation. If the transducer is based on a variation in electrical property, the
first stage must convert that variation in electrical property into a variation in
voltage. If the transducer element is single ended, i.e., only one element changes,
then a constant current source can be used and the detector equation follows
ohm’s law:
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
27.
Vout = I(Z + ∆Z) where ∆Z = f(input energy). (1)
Figure 1.2 shows an example of a single transducer element used in opera-
tional amplifier circuit that provides constant current operation. The transducer
element in this case is a thermistor, an element that changes its resistance with
temperature. Using circuit analysis, it is easy to show that the thermistor is
driven by a constant current of VS /R amps. The output, Vout, is [(RT + ∆RT)/R]VS.
Alternatively, an approximate constant current source can be generated using a
voltage source and a large series resistor, RS, where RS >> ∆R.
If the transducer can be configured differentially so that one element in-
creases with increasing input energy while the other element decreases, the
bridge circuit is commonly used as a detector. Figure 1.3 shows a device made
to measure intestinal motility using strain gages. A bridge circuit detector is
used in conjunction with a pair of differentially configured strain gages: when
the intestine contracts, the end of the cantilever beam moves downward and the
upper strain gage (visible) is stretched and increases in resistance while the
lower strain gage (not visible) compresses and decreases in resistance. The out-
put of the bridge circuit can be found from simple circuit analysis to be: Vout =
VS∆R/2, where VS is the value of the source voltage. If the transducer operates
based on a change in inductance or capacitance, the above techniques are still
useful except a sinusoidal voltage source must be used.
If the transducer element is a voltage generator, the first stage is usually
an amplifier. If the transducer produces a current output, as is the case in many
electromagnetic detectors, then a current-to-voltage amplifier (also termed a
transconductance amplifier) is used to produce a voltage output.
FIGURE 1.2 A thermistor (a semiconductor that changes resistance as a function
of temperature) used in a constant current configuration.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
28.
FIGURE 1.3 A strain gage probe used to measure motility of the intestine. The
bridge circuit is used to convert differential change in resistance from a pair of
strain gages into a change in voltage.
Figure 1.4 shows a photodiode transducer used with a transconductance
amplifier. The output voltage is proportional to the current through the photodi-
ode: Vout = RfIdiode. Bandwidth can be increased at the expense of added noise by
reverse biasing the photodiode with a small voltage.* More sophisticated detec-
tion systems such as phase sensitive detectors (PSD) can be employed in some
cases to improve noise rejection. A software implementation of PSD is de-
scribed in Chapter 8. In a few circumstances, additional amplification beyond
the first stage may be required.
SOURCES OF VARIABILITY: NOISE
In this text, noise is a very general and somewhat relative term: noise is what
you do not want and signal is what you do want. Noise is inherent in most
measurement systems and often the limiting factor in the performance of a medi-
cal instrument. Indeed, many signal processing techniques are motivated by the
*A bias voltage improves movement of charge through the diode decreasing the response time.
From −10 to −50 volts are used, except in the case of avalanche photodiodes where a higher voltage
is required.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
29.
FIGURE 1.4 Photodiode used in a transconductance amplifier.
desire to minimize the variability in the measurement. In biomedical measure-
ments, variability has four different origins: (1) physiological variability; (2) en-
vironmental noise or interference; (3) transducer artifact; and (4) electronic noise.
Physiological variability is due to the fact that the information you desire is based
on a measurement subject to biological influences other than those of interest.
For example, assessment of respiratory function based on the measurement of
blood pO2 could be confounded by other physiological mechanisms that alter
blood pO2. Physiological variability can be a very difficult problem to solve,
sometimes requiring a totally different approach.
Environmental noise can come from sources external or internal to the
body. A classic example is the measurement of fetal ECG where the desired
signal is corrupted by the mother’s ECG. Since it is not possible to describe the
specific characteristics of environmental noise, typical noise reduction tech-
niques such as filtering are not usually successful. Sometimes environmental
noise can be reduced using adaptive techniques such as those described in Chap-
ter 8 since these techniques do not require prior knowledge of noise characteris-
tics. Indeed, one of the approaches described in Chapter 8, adaptive noise can-
cellation, was initially developed to reduce the interference from the mother in
the measurement of fetal ECG.
Transducer artifact is produced when the transducer responds to energy
modalities other than that desired. For example, recordings of electrical poten-
tials using electrodes placed on the skin are sensitive to motion artifact, where
the electrodes respond to mechanical movement as well as the desired electrical
signal. Transducer artifacts can sometimes be successfully addressed by modifi-
cations in transducer design. Aerospace research has led to the development of
electrodes that are quite insensitive to motion artifact.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
30.
Unlike the other sources of variability, electronic noise has well-known
sources and characteristics. Electronic noise falls into two broad classes: thermal
or Johnson noise, and shot noise. The former is produced primarily in resistor
or resistance materials while the latter is related to voltage barriers associated
with semiconductors. Both sources produce noise with a broad range of frequen-
cies often extending from DC to 1012
–1013
Hz. Such a broad spectrum noise is
referred to as white noise since it contains energy at all frequencies (or at least
all the frequencies of interest to biomedical engineers). Figure 1.5 shows a plot
of power density versus frequency for white noise calculated from a noise wave-
form (actually an array of random numbers) using the spectra analysis methods
described in Chapter 3. Note that its energy is fairly constant across the spectral
range.
The various sources of noise or variability along with their causes and
possible remedies are presented in Table 1.2 below. Note that in three out of
four instances, appropriate transducer design was useful in the reduction of the
FIGURE 1.5 Power density (power spectrum) of digitizied white noise showing a
fairly constant value over frequency.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
31.
TABLE 1.2 Sources of Variability
Source Cause Potential Remedy
Physiological Measurement only indi- Modify overall approach
variability rectly related to variable
of interest
Environmental Other sources of similar Noise cancellation
(internal or external) energy form Transducer design
Artifact Transducer responds to Transducer design
other energy sources
Electronic Thermal or shot noise Transducer or electronic
design
variability or noise. This demonstrates the important role of the transducer in
the overall performance of the instrumentation system.
Electronic Noise
Johnson or thermal noise is produced by resistance sources, and the amount of
noise generated is related to the resistance and to the temperature:
VJ = √4kT R B volts (2)
where R is the resistance in ohms, T the temperature in degrees Kelvin, and k
is Boltzman’s constant (k = 1.38 × 10−23
J/°K).* B is the bandwidth, or range of
frequencies, that is allowed to pass through the measurement system. The sys-
tem bandwidth is determined by the filter characteristics in the system, usually
the analog filtering in the system (see the next section).
If noise current is of interest, the equation for Johnson noise current can
be obtained from Eq. (2) in conjunction with Ohm’s law:
IJ = √4kT B/R amps (3)
Since Johnson noise is spread evenly over all frequencies (at least in the-
ory), it is not possible to calculate a noise voltage or current without specifying
B, the frequency range. Since the bandwidth is not always known in advance, it
is common to describe a relative noise; specifically, the noise that would occur
if the bandwidth were 1.0 Hz. Such relative noise specification can be identified
by the unusual units required: volts/√Hz or amps/√Hz.
*A temperature of 310 °K is often used as room temperature, in which case 4kT = 1.7 × 10−20
J.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
32.
Shot noise is defined as a current noise and is proportional to the baseline
current through a semiconductor junction:
Is = √2q Id B amps (4)
where q is the charge on an electron (1.662 × 10−19
coulomb), and Id is the
baseline semiconductor current. In photodetectors, the baseline current that gen-
erates shot noise is termed the dark current, hence, the symbol Id in Eq. (4).
Again, since the noise is spread across all frequencies, the bandwidth, BW, must
be specified to obtain a specific value, or a relative noise can be specified in
amps/√Hz.
When multiple noise sources are present, as is often the case, their voltage
or current contributions to the total noise add as the square root of the sum of
the squares, assuming that the individual noise sources are independent. For
voltages:
VT = (V2
1 + V2
2 + V2
3 + ؒ ؒ ؒ + V2
N)1/2
(5)
A similar equation applies to current. Noise properties are discussed fur-
ther in Chapter 2.
Signal-to-Noise Ratio
Most waveforms consist of signal plus noise mixed together. As noted pre-
viously, signal and noise are relative terms, relative to the task at hand: the
signal is that portion of the waveform of interest while the noise is everything
else. Often the goal of signal processing is to separate out signal from noise, to
identify the presence of a signal buried in noise, or to detect features of a signal
buried in noise.
The relative amount of signal and noise present in a waveform is usually
quantified by the signal-to-noise ratio, SNR. As the name implies, this is simply
the ratio of signal to noise, both measured in RMS (root-mean-squared) ampli-
tude. The SNR is often expressed in "db" (short for decibels) where:
SNR = 20 log ͩSignal
Noiseͪ (6)
To convert from db scale to a linear scale:
SNRlinear = 10db/20
(7)
For example, a ratio of 20 db means that the RMS value of the signal was
10 times the RMS value of the noise (1020/20
= 10), +3 db indicates a ratio of
1.414 (103/20
= 1.414), 0 db means the signal and noise are equal in RMS value,
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
33.
−3 db means that the ratio is 1/1.414, and −20 db means the signal is 1/10 of
the noise in RMS units. Figure 1.6 shows a sinusoidal signal with various
amounts of white noise. Note that is it is difficult to detect presence of the signal
visually when the SNR is −3 db, and impossible when the SNR is −10 db. The
ability to detect signals with low SNR is the goal and motivation for many of
the signal processing tools described in this text.
ANALOG FILTERS: FILTER BASICS
The analog signal processing circuitry shown in Figure 1.1 will usually contain
some filtering, both to remove noise and appropriately condition the signal for
FIGURE 1.6 A 30 Hz sine wave with varying amounts of added noise. The sine
wave is barely discernable when the SNR is −3db and not visible when the SNR
is −10 db.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
34.
analog-to-digital conversion (ADC). It is this filtering that usually establishes
the bandwidth of the system for noise calculations [the bandwidth used in Eqs.
(2)–(4)]. As shown later, accurate conversion of the analog signal to digital
format requires that the signal contain frequencies no greater than 1⁄2 the sam-
pling frequency. This rule applies to the analog waveform as a whole, not just
the signal of interest. Since all transducers and electronics produce some noise
and since this noise contains a wide range of frequencies, analog lowpass filter-
ing is usually essential to limit the bandwidth of the waveform to be converted.
Waveform bandwidth and its impact on ADC will be discussed further in Chap-
ter 2. Filters are defined by several properties: filter type, bandwidth, and attenu-
ation characteristics. The last can be divided into initial and final characteristics.
Each of these properties is described and discussed in the next section.
Filter Types
Analog filters are electronic devices that remove selected frequencies. Filters
are usually termed according to the range of frequencies they do not suppress.
Thus, lowpass filters allow low frequencies to pass with minimum attenuation
while higher frequencies are attenuated. Conversely, highpass filters pass high
frequencies, but attenuate low frequencies. Bandpass filters reject frequencies
above and below a passband region. An exception to this terminology is the
bandstop filter, which passes frequencies on either side of a range of attenuated
frequencies.
Within each class, filters are also defined by the frequency ranges that
they pass, termed the filter bandwidth, and the sharpness with which they in-
crease (or decrease) attenuation as frequency varies. Spectral sharpness is speci-
fied in two ways: as an initial sharpness in the region where attenuation first
begins and as a slope further along the attenuation curve. These various filter
properties are best described graphically in the form of a frequency plot (some-
times referred to as a Bode plot), a plot of filter gain against frequency. Filter
gain is simply the ratio of the output voltage divided by the input voltage, Vout/
Vin, often taken in db. Technically this ratio should be defined for all frequencies
for which it is nonzero, but practically it is usually stated only for the frequency
range of interest. To simplify the shape of the resultant curves, frequency plots
sometimes plot gain in db against the log of frequency.* When the output/input
ratio is given analytically as a function of frequency, it is termed the transfer
function. Hence, the frequency plot of a filter’s output/input relationship can be
*When gain is plotted in db, it is in logarithmic form, since the db operation involves taking the
log [Eq. (6)]. Plotting gain in db against log frequency puts the two variables in similar metrics and
results in straighter line plots.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
35.
viewed as a graphical representation of the transfer function. Frequency plots
for several different filter types are shown in Figure 1.7.
Filter Bandwidth
The bandwidth of a filter is defined by the range of frequencies that are not
attenuated. These unattenuated frequencies are also referred to as passband fre-
quencies. Figure 1.7A shows that the frequency plot of an ideal filter, a filter
that has a perfectly flat passband region and an infinite attenuation slope. Real
filters may indeed be quite flat in the passband region, but will attenuate with a
FIGURE 1.7 Frequency plots of ideal and realistic filters. The frequency plots
shown here have a linear vertical axis, but often the vertical axis is plotted in db.
The horizontal axis is in log frequency. (A) Ideal lowpass filter. (B) Realistic low-
pass filter with a gentle attenuation characteristic. (C) Realistic lowpass filter with
a sharp attenuation characteristic. (D) Bandpass filter.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
36.
more gentle slope, as shown in Figure 1.7B. In the case of the ideal filter, Figure
1.7A, the bandwidth or region of unattenuated frequencies is easy to determine;
specifically, it is between 0.0 and the sharp attenuation at fc Hz. When the
attenuation begins gradually, as in Figure 1.7B, defining the passband region is
problematic. To specify the bandwidth in this filter we must identify a frequency
that defines the boundary between the attenuated and non-attenuated portion of
the frequency characteristic. This boundary has been somewhat arbitrarily de-
fined as the frequency when the attenuation is 3 db.* In Figure 1.7B, the filter
would have a bandwidth of 0.0 to fc Hz, or simply fc Hz. The filter in Figure
1.7C has a sharper attenuation characteristic, but still has the same bandwidth
( fc Hz). The bandpass filter of Figure 1.7D has a bandwidth of fh − fl Hz.
Filter Order
The slope of a filter’s attenuation curve is related to the complexity of the filter:
more complex filters have a steeper slope better approaching the ideal. In analog
filters, complexity is proportional to the number of energy storage elements in
the circuit (which could be either inductors or capacitors, but are generally ca-
pacitors for practical reasons). Using standard circuit analysis, it can be shown
that each energy storage device leads to an additional order in the polynomial
of the denominator of the transfer function that describes the filter. (The denom-
inator of the transfer function is also referred to as the characteristic equation.)
As with any polynomial equation, the number of roots of this equation will
depend on the order of the equation; hence, filter complexity (i.e., the number
of energy storage devices) is equivalent to the number of roots in the denomina-
tor of the Transfer Function. In electrical engineering, it has long been common
to call the roots of the denominator equation poles. Thus, the complexity of the
filter is also equivalent to the number of poles in the transfer function. For
example, a second-order or two-pole filter has a transfer function with a second-
order polynomial in the denominator and would contain two independent energy
storage elements (very likely two capacitors).
Applying asymptote analysis to the transfer function, is not difficult to
show that the slope of a second-order lowpass filter (the slope for frequencies
much greater than the cutoff frequency, fc) is 40 db/decade specified in log-log
terms. (The unusual units, db/decade are a result of the log-log nature of the
typical frequency plot.) That is, the attenuation of this filter increases linearly
on a log-log scale by 40 db (a factor of 100 on a linear scale) for every order
of magnitude increase in frequency. Generalizing, for each filter pole (or order)
*This defining point is not entirely arbitrary because when the signal is attenuated 3 db, its ampli-
tude is 0.707 (10−3/20
) of what it was in the passband region and it has half the power of the unattenu-
ated signal (since 0.7072
= 1/2). Accordingly this point is also known as the half-power point.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
37.
the downward slope (sometimes referred to as the rolloff) is increased by 20
db/decade. Figure 1.8 shows the frequency plot of a second-order (two-pole
with a slope of 40 db/decade) and a 12th-order lowpass filter, both having the
same cutoff frequency, fc, and hence, the same bandwidth. The steeper slope or
rolloff of the 12-pole filter is apparent. In principle, a 12-pole lowpass filter
would have a slope of 240 db/decade (12 × 20 db/decade). In fact, this fre-
quency characteristic is theoretical because in real analog filters parasitic com-
ponents and inaccuracies in the circuit elements limit the actual attenuation that
can be obtained. The same rationale applies to highpass filters except that the
frequency plot decreases with decreasing frequency at a rate of 20 db/decade
for each highpass filter pole.
Filter Initial Sharpness
As shown in Figure 1.8, both the slope and the initial sharpness increase with
filter order (number of poles), but increasing filter order also increases the com-
FIGURE 1.8 Frequency plot of a second-order (2-pole) and a 12th-order lowpass
filter with the same cutoff frequency. The higher order filter more closely ap-
proaches the sharpness of an ideal filter.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
38.
plexity, hence the cost, of the filter. It is possible to increase the initial sharpness
of the filter’s attenuation characteristics without increasing the order of the filter,
if you are willing to except some unevenness, or ripple, in the passband. Figure
1.9 shows two lowpass, 4th
-order filters, differing in the initial sharpness of the
attenuation. The one marked Butterworth has a smooth passband, but the initial
attenuation is not as sharp as the one marked Chebychev; which has a passband
that contains ripples. This property of analog filters is also seen in digital filters
and will be discussed in detail in Chapter 4.
FIGURE 1.9 Two filters having the same order (4-pole) and cutoff frequency, but
differing in the sharpness of the initial slope. The filter marked Chebychev has a
steeper initial slope or rolloff, but contains ripples in the passband.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
39.
ANALOG-TO-DIGITAL CONVERSION: BASIC CONCEPTS
The last analog element in a typical measurement system is the analog-to-digital
converter (ADC), Figure 1.1. As the name implies, this electronic component
converts an analog voltage to an equivalent digital number. In the process of
analog-to-digital conversion an analog or continuous waveform, x(t), is con-
verted into a discrete waveform, x(n), a function of real numbers that are defined
only at discrete integers, n. To convert a continuous waveform to digital format
requires slicing the signal in two ways: slicing in time and slicing in amplitude
(Figure 1.10).
Slicing the signal into discrete points in time is termed time sampling or
simply sampling. Time slicing samples the continuous waveform, x(t), at dis-
crete prints in time, nTs, where Ts is the sample interval. The consequences of
time slicing are discussed in the next chapter. The same concept can be applied
to images wherein a continuous image such as a photograph that has intensities
that vary continuously across spatial distance is sampled at distances of S mm.
In this case, the digital representation of the image is a two-dimensional array.
The consequences of spatial sampling are discussed in Chapter 11.
Since the binary output of the ADC is a discrete integer while the analog
signal has a continuous range of values, analog-to-digital conversion also re-
quires the analog signal to be sliced into discrete levels, a process termed quanti-
zation, Figure 1.10. The equivalent number can only approximate the level of
FIGURE 1.10 Converting a continuous signal (solid line) to discrete format re-
quires slicing the signal in time and amplitude. The result is a series of discrete
points (X’s) that approximate the original signal.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
40.
the analog signal, and the degree of approximation will depend on the range of
binary numbers and the amplitude of the analog signal. For example, if the
output of the ADC is an 8-bit binary number capable of 28
or 256 discrete states,
and the input amplitude range is 0.0–5.0 volts, then the quantization interval
will be 5/256 or 0.0195 volts. If, as is usually the case, the analog signal is time
varying in a continuous manner, it must be approximated by a series of binary
numbers representing the approximate analog signal level at discrete points in
time (Figure 1.10). The errors associated with amplitude slicing, or quantization,
are described in the next section, and the potential error due to sampling is
covered in Chapter 2. The remainder of this section briefly describes the hard-
ware used to achieve this approximate conversion.
Analog-to-Digital Conversion Techniques
Various conversion rules have been used, but the most common is to convert
the voltage into a proportional binary number. Different approaches can be used
to implement the conversion electronically; the most common is the successive
approximation technique described at the end of this section. ADC’s differ in
conversion range, speed of conversion, and resolution. The range of analog volt-
ages that can be converted is frequently software selectable, and may, or may
not, include negative voltages. Typical ranges are from 0.0–10.0 volts or less,
or if negative values are possible ± 5.0 volts or less. The speed of conversion
is specified in terms of samples per second, or conversion time. For example,
an ADC with a conversion time of 10 µsec should, logically, be able to operate
at up to 100,000 samples per second (or simply 100 kHz). Typical conversion
rates run up to 500 kHz for moderate cost converters, but off-the-shelf converters
can be obtained with rates up to 10–20 MHz. Except for image processing
systems, lower conversion rates are usually acceptable for biological signals.
Even image processing systems may use downsampling techniques to reduce
the required ADC conversion rate and, hence, the cost.
A typical ADC system involves several components in addition to the
actual ADC element, as shown in Figure 1.11. The first element is an N-to-1
analog switch that allows multiple input channels to be converted. Typical ADC
systems provide up to 8 to 16 channels, and the switching is usually software-
selectable. Since a single ADC is doing the conversion for all channels, the
conversion rate for any given channel is reduced in proportion to the number of
channels being converted. Hence, an ADC system with converter element that
had a conversion rate of 50 kHz would be able to sample each of eight channels
at a theoretical maximum rate of 50/8 = 6.25 kHz.
The Sample and Hold is a high-speed switch that momentarily records the
input signal, and retains that signal value at its output. The time the switch is
closed is termed the aperture time. Typical values range around 150 ns, and,
except for very fast signals, can be considered basically instantaneous. This
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
41.
FIGURE 1.11 Block diagram of a typical analog-to-digital conversion system.
instantaneously sampled voltage value is held (as a charge on a capacitor) while
the ADC element determines the equivalent binary number. Again, it is the
ADC element that determines the overall speed of the conversion process.
Quantization Error
Resolution is given in terms of the number of bits in the binary output with the
assumption that the least significant bit (LSB) in the output is accurate (which
may not always be true). Typical converters feature 8-, 12-, and 16-bit output
with 12 bits presenting a good compromise between conversion resolution and
cost. In fact, most signals do not have a sufficient signal-to-noise ratio to justify
a higher resolution; you are simply obtaining a more accurate conversion of the
noise. For example, assuming that converter resolution is equivalent to the LSB,
then the minimum voltage that can be resolved is the same as the quantization
voltage described above: the voltage range divided by 2N
, where N is the number
of bits in the binary output. The resolution of a 5-volt, 12-bit ADC is 5.0/212
=
5/4096 = 0.0012 volts. The dynamic range of a 12-bit ADC, the range from the
smallest to the largest voltage it can convert, is from 0.0012 to 5 volts: in db
this is 20 * log*1012
* = 167 db. Since typical signals, especially those of biologi-
cal origin, have dynamic ranges rarely exceeding 60 to 80 db, a 12-bit converter
with the dynamic range of 167 db may appear to be overkill. However, having
this extra resolution means that not all of the range need be used, and since 12-
bit ADC’s are only marginally more expensive than 8-bit ADC’s they are often
used even when an 8-bit ADC (with dynamic range of over 100 DB, would be
adequate). A 12-bit output does require two bytes to store and will double the
memory requirements over an 8-bit ADC.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
42.
The number of bits used for conversion sets a lower limit on the resolu-
tion, and also determines the quantization error (Figure 1.12). This error can be
thought of as a noise process added to the signal. If a sufficient number of
quantization levels exist (say N > 64), the distortion produced by quantization
error may be modeled as additive independent white noise with zero mean with
the variance determined by the quantization step size, δ = VMAX/2N
. Assuming
that the error is uniformly distributed between −δ/2 +δ/2, the variance, σ, is:
σ = ∫
δ/2
−δ/2
η2
/δ dη = V2
Max (2−2N
)/12 (8)
Assuming a uniform distribution, the RMS value of the noise would be
just twice the standard deviation, σ.
Further Study: Successive Approximation
The most popular analog-to-digital converters use a rather roundabout strategy
to find the binary number most equivalent to the input analog voltage—a digi-
tal-to-analog converter (DAC) is placed in a feedback loop. As shown Figure
1.13, an initial binary number stored in the buffer is fed to a DAC to produce a
FIGURE 1.12 Quantization (amplitude slicing) of a continuous waveform. The
lower trace shows the error between the quantized signal and the input.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
43.
FIGURE 1.13 Block diagram of an analog-to-digital converter. The input analog
voltage is compared with the output of a digital-to-analog converter. When the
two voltages match, the number held in the binary buffer is equivalent to the input
voltage with the resolution of the converter. Different strategies can be used to
adjust the contents of the binary buffer to attain a match.
proportional voltage, VDAC. This DAC voltage, VDAC, is then compared to the
input voltage, and the binary number in the buffer is adjusted until the desired
level of match between VDAC and Vin is obtained. This approach begs the question
“How are DAC’s constructed?” In fact, DAC’s are relatively easy to construct
using a simple ladder network and the principal of current superposition.
The controller adjusts the binary number based on whether or not the
comparator finds the voltage out of the DAC, VDAC, to be greater or less than
the input voltage, Vin. One simple adjustment strategy is to increase the binary
number by one each cycle if VDAC < Vin, or decrease it otherwise. This so-called
tracking ADC is very fast when Vin changes slowly, but can take many cycles
when Vin changes abruptly (Figure 1.14). Not only can the conversion time be
quite long, but it is variable since it depends on the dynamics of the input signal.
This strategy would not easily allow for sampling an analog signal at a fixed
rate due to the variability in conversion time.
An alternative strategy termed successive approximation allows the con-
version to be done at a fixed rate and is well-suited to digital technology. The
successive approximation strategy always takes the same number of cycles irre-
spective of the input voltage. In the first cycle, the controller sets the most
significant bit (MSB) of the buffer to 1; all others are cleared. This binary
number is half the maximum possible value (which occurs when all the bits are
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
44.
FIGURE 1.14 Voltage waveform of an ADC that uses a tracking strategy. The
ADC voltage (solid line) follows the input voltage (dashed line) fairly closely when
the input voltage varies slowly, but takes many cycles to “catch up” to an abrupt
change in input voltage.
1), so the DAC should output a voltage that is half its maximum voltage—that
is, a voltage in the middle of its range. If the comparator tells the controller that
Vin > VDAC, then the input voltage, Vin, must be greater than half the maximum
range, and the MSB is left set. If Vin < VDAC, then that the input voltage is in the
lower half of the range and the MSB is cleared (Figure 1.15). In the next cycle,
the next most significant bit is set, and the same comparison is made and the
same bit adjustment takes place based on the results of the comparison (Figure
1.15).
After N cycles, where N is the number of bits in the digital output, the
voltage from the DAC, VDAC, converges to the best possible fit to the input
voltage, Vin. Since Vin Ϸ VDAC, the number in the buffer, which is proportional
to VDAC, is the best representation of the analog input voltage within the resolu-
tion of the converter. To signal the end of the conversion process, the ADC puts
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
45.
FIGURE 1.15 Vin and VDAC in a 6-bit ADC using the successive approximation
strategy. In the first cycle, the MSB is set (solid line) since Vin > VDAC . In the next
two cycles, the bit being tested is cleared because Vin < VDAC when this bit was
set. For the fourth and fifth cycles the bit being tested remained set and for the
last cycle it was cleared. At the end of the sixth cycle a conversion complete flag
is set to signify the end of the conversion process.
out a digital signal or flag indicating that the conversion is complete (Figure
1.15).
TIME SAMPLING: BASICS
Time sampling transforms a continuous analog signal into a discrete time signal,
a sequence of numbers denoted as x(n) = [x1, x2, x3, . . . xN],* Figure 1.16 (lower
trace). Such a representation can be thought of as an array in computer memory.
(It can also be viewed as a vector as shown in the next chapter.) Note that the
array position indicates a relative position in time, but to relate this number
sequence back to an absolute time both the sampling interval and sampling onset
time must be known. However, if only the time relative to conversion onset is
important, as is frequently the case, then only the sampling interval needs to be
*In many textbooks brackets, [ ], are used to denote digitized variables; i.e., x[n]. Throughout this
text we reserve brackets to indicate a series of numbers, or vector, following the MATLAB format.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
46.
FIGURE 1.16 A continuous signal (upper trace) is sampled at discrete points in
time and stored in memory as an array of proportional numbers (lower trace).
known. Converting back to relative time is then achieved by multiplying the
sequence number, n, by the sampling interval, Ts: x(t) = x(nTs).
Sampling theory is discussed in the next chapter and states that a sinusoid
can be uniquely reconstructed providing it has been sampled by at least two
equally spaced points over a cycle. Since Fourier series analysis implies that
any signal can be represented is a series of sin waves (see Chapter 3), then by
extension, a signal can be uniquely reconstructed providing the sampling fre-
quency is twice that of the highest frequency in the signal. Note that this highest
frequency component may come from a noise source and could be well above
the frequencies of interest. The inverse of this rule is that any signal that con-
tains frequency components greater than twice the sampling frequency cannot
be reconstructed, and, hence, its digital representation is in error. Since this error
is introduced by undersampling, it is inherent in the digital representation and
no amount of digital signal processing can correct this error. The specific nature
of this under-sampling error is termed aliasing and is described in a discussion
of the consequences of sampling in Chapter 2.
From a practical standpoint, aliasing must be avoided either by the use of
very high sampling rates—rates that are well above the bandwidth of the analog
system—or by filtering the analog signal before analog-to-digital conversion.
Since extensive sampling rates have an associated cost, both in terms of the
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
47.
ADC required and memory costs, the latter approach is generally preferable.
Also note that the sampling frequency must be twice the highest frequency
present in the input signal, not to be confused with the bandwidth of the analog
signal. All frequencies in the sampled waveform greater than one half the sam-
pling frequency (one-half the sampling frequency is sometimes referred to as
the Nyquist frequency) must be essentially zero, not merely attenuated. Recall
that the bandwidth is defined as the frequency for which the amplitude is re-
duced by only 3 db from the nominal value of the signal, while the sampling
criterion requires that the value be reduced to zero. Practically, it is sufficient
to reduce the signal to be less than quantization noise level or other acceptable
noise level. The relationship between the sampling frequency, the order of the
anti-aliasing filter, and the system bandwidth is explored in a problem at the
end of this chapter.
Example 1.1. An ECG signal of 1 volt peak-to-peak has a bandwidth of
0.01 to 100 Hz. (Note this frequency range has been established by an official
standard and is meant to be conservative.) Assume that broadband noise may
be present in the signal at about 0.1 volts (i.e., −20 db below the nominal signal
level). This signal is filtered using a four-pole lowpass filter. What sampling
frequency is required to insure that the error due to aliasing is less than −60 db
(0.001 volts)?
Solution. The noise at the sampling frequency must be reduced another
40 db (20 * log (0.1/0.001)) by the four-pole filter. A four-pole filter with a
cutoff of 100 Hz (required to meet the fidelity requirements of the ECG signal)
would attenuate the waveform at a rate of 80 db per decade. For a four-pole
filter the asymptotic attenuation is given as:
Attenuation = 80 log(f2/fc) db
To achieve the required additional 40 db of attenuation required by the
problem from a four-pole filter:
80 log(f2/fc) = 40 log(f2/fc) = 40/80 = 0.5
f2/fc = 10.5 =; f2 = 3.16 × 100 = 316 Hz
Thus to meet the sampling criterion, the sampling frequency must be at
least 632 Hz, twice the frequency at which the noise is adequately attenuated.
The solution is approximate and ignores the fact that the initial attenuation of
the filter will be gradual. Figure 1.17 shows the frequency response characteris-
tics of an actual 4-pole analog filter with a cutoff frequency of 100 Hz. This
figure shows that the attenuation is 40 db at approximately 320 Hz. Note the
high sampling frequency that is required for what is basically a relatively low
frequency signal (the ECG). In practice, a filter with a sharper cutoff, perhaps
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
48.
FIGURE 1.17 Detailed frequency plot (on a log-log scale) of a 4-pole and 8-pole
filter, both having a cutoff frequency of 100 Hz.
an 8-pole filter, would be a better choice in this situation. Figure 1.17 shows
that the frequency response of an 8-pole filter with the same 100 Hz frequency
provides the necessary attenuation at less than 200 Hz. Using this filter, the
sampling frequency could be lowered to under 400 Hz.
FURTHER STUDY: BUFFERING
AND REAL-TIME DATA PROCESSING
Real-time data processing simply means that the data is processed and results
obtained in sufficient time to influence some ongoing process. This influence
may come directly from the computer or through human intervention. The pro-
cessing time constraints naturally depend on the dynamics of the process of
interest. Several minutes might be acceptable for an automated drug delivery
system, while information on the electrical activity the heart needs to be imme-
diately available.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
49.
The term buffer, when applied digital technology, usually describes a set
of memory locations used to temporarily store incoming data until enough data
is acquired for efficient processing. When data is being acquired continuously,
a technique called double buffering can be used. Incoming data is alternatively
sent to one of two memory arrays, and the one that is not being filled is pro-
cessed (which may involve simply transfer to disk storage). Most ADC software
packages provide a means for determining which element in an array has most
recently been filled to facilitate buffering, and frequently the ability to determine
which of two arrays (or which half of a single array) is being filled to facilitate
double buffering.
DATA BANKS
With the advent of the World Wide Web it is not always necessary to go through
the analog-to-digital conversion process to obtain digitized data of physiological
signals. A number of data banks exist that provide physiological signals such as
ECG, EEG, gait, and other common biosignals in digital form. Given the volatil-
ity and growth of the Web and the ease with which searches can be made, no
attempt will be made to provide a comprehensive list of appropriate Websites.
However, a good source of several common biosignals, particularly the ECG, is
the Physio Net Data Bank maintained by MIT—http://www.physionet.org. Some
data banks are specific to a given set of biosignals or a given signal processing
approach. An example of the latter is the ICALAB Data Bank in Japan—http://
www.bsp.brain.riken.go.jp/ICALAB/—which includes data that can be used to
evaluate independent component analysis (see Chapter 9) algorithms.
Numerous other data banks containing biosignals and/or images can be
found through a quick search of the Web, and many more are likely to come
online in the coming years. This is also true for some of the signal processing
algorithms as will be described in more detail later. For example, the ICALAB
Website mentioned above also has algorithms for independent component analy-
sis in MATLAB m-file format. A quick Web search can provide both signal
processing algorithms and data that can be used to evaluate a signal processing
system under development. The Web is becoming an evermore useful tool in
signal and image processing, and a brief search of the Web can save consider-
able time in the development process, particularly if the signal processing sys-
tem involves advanced approaches.
PROBLEMS
1. A single sinusoidal signal is contained in noise. The RMS value of the noise
is 0.5 volts and the SNR is 10 db. What is the peak-to-peak amplitude of the
sinusoid?
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
50.
2. A resistor produces 10 µV noise when the room temperature is 310°K and
the bandwidth is 1 kHz. What current noise would be produced by this resistor?
3. The noise voltage out of a 1 MΩ resistor was measured using a digital volt
meter as 1.5 µV at a room temperature of 310 °K. What is the effective band-
width of the voltmeter?
4. The photodetector shown in Figure 1.4 has a sensitivity of 0.3µA/µW (at a
wavelength of 700 nm). In this circuit, there are three sources of noise. The
photodetector has a dark current of 0.3 nA, the resistor is 10 MΩ, and the
amplifier has an input current noise of 0.01 pA/√Hz. Assume a bandwidth of
10 kHz. (a) Find the total noise current input to the amplifier. (b) Find the
minimum light flux signal that can be detected with an SNR = 5.
5. A lowpass filter is desired with the cutoff frequency of 10 Hz. This filter
should attenuate a 100 Hz signal by a factor of 85. What should be the order of
this filter?
6. You are given a box that is said to contain a highpass filter. You input a
series of sine waves into the box and record the following output:
Frequency (Hz): 2 10 20 60 100 125 150 200 300 400
Vout volts rms: .15×10−7
0.1×10−3
0.002 0.2 1.5 3.28 4.47 4.97 4.99 5.0
What is the cutoff frequency and order of this filter?
7. An 8-bit ADC converter that has an input range of ± 5 volts is used to
convert a signal that varies between ± 2 volts. What is the SNR of the input if
the input noise equals the quantization noise of the converter?
8. As elaborated in Chapter 2, time sampling requires that the maximum fre-
quency present in the input be less than fs/2 for proper representation in digital
format. Assume that the signal must be attenuated by a factor of 1000 to be
considered “not present.” If the sampling frequency is 10 kHz and a 4th-order
lowpass anti-aliasing filter is used prior to analog-to-digital conversion, what
should be the bandwidth of the sampled signal? That is, what must the cutoff
frequency be of the anti-aliasing lowpass filter?
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
51.
10
Fundamentals of Image Processing:
MATLAB Image Processing Toolbox
IMAGE PROCESSING BASICS: MATLAB IMAGE FORMATS
Images can be treated as two-dimensional data, and many of the signal process-
ing approaches presented in the previous chapters are equally applicable to im-
ages: some can be directly applied to image data while others require some
modification to account for the two (or more) data dimensions. For example,
both PCA and ICA have been applied to image data treating the two-dimen-
sional image as a single extended waveform. Other signal processing methods
including Fourier transformation, convolution, and digital filtering are applied to
images using two-dimensional extensions. Two-dimensional images are usually
represented by two-dimensional data arrays, and MATLAB follows this tradi-
tion;* however, MATLAB offers a variety of data formats in addition to the
standard format used by most MATLAB operations. Three-dimensional images
can be constructed using multiple two-dimensional representations, but these
multiple arrays are sometimes treated as a single volume image.
General Image Formats: Image Array Indexing
Irrespective of the image format or encoding scheme, an image is always repre-
sented in one, or more, two dimensional arrays, I(m,n). Each element of the
*Actually, MATLAB considers image data arrays to be three-dimensional, as described later in this
chapter.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
52.
variable, I, represents a single picture element, or pixel. (If the image is being
treated as a volume, then the element, which now represents an elemental vol-
ume, is termed a voxel.) The most convenient indexing protocol follows the
traditional matrix notation, with the horizontal pixel locations indexed left to
right by the second integer, n, and the vertical locations indexed top to bottom
by the first integer m (Figure 10.1). This indexing protocol is termed pixel coor-
dinates by MATLAB. A possible source of confusion with this protocol is that
the vertical axis positions increase from top to bottom and also that the second
integer references the horizontal axis, the opposite of conventional graphs.
MATLAB also offers another indexing protocol that accepts non-integer
indexes. In this protocol, termed spatial coordinates, the pixel is considered to
be a square patch, the center of which has an integer value. In the default coordi-
nate system, the center of the upper left-hand pixel still has a reference of (1,1),
but the upper left-hand corner of this pixel has coordinates of (0.5,0.5) (see
Figure 10.2). In this spatial coordinate system, the locations of image coordi-
nates are positions on a (discrete) plane and are described by general variables
x and y. The are two sources of potential confusion with this system. As with
the pixel coordinate system, the vertical axis increases downward. In addition,
the positions of the vertical and horizontal indexes (now better though of as
coordinates) are switched: the horizontal index is first, followed by the vertical
coordinate, as with conventional x,y coordinate references. In the default spatial
coordinate system, integer coordinates correspond with their pixel coordinates,
remembering the position swap, so that I(5,4) in pixel coordinates references
the same pixel as I(4.0,5.0) in spatial coordinates. Most routines expect a
specific pixel coordinate system and produce outputs in that system. Examples
of spatial coordinates are found primarily in the spatial transformation routines
described in the next chapter.
It is possible to change the baseline reference in the spatial coordinate
FIGURE 10.1 Indexing format for MATLAB images using the pixel coordinate sys-
tem. This indexing protocol follows the standard matrix notation.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
53.
FIGURE 10.2 Indexing in the spatial coordinate system.
system as certain commands allow you to redefine the coordinates of the refer-
ence corner. This option is described in context with related commands.
Data Classes: Intensity Coding Schemes
There are four different data classes, or encoding schemes, used by MATLAB
for image representation. Moreover, each of these data classes can store the data
in a number of different formats. This variety reflects the variety in image types
(color, grayscale, and black and white), and the desire to represent images as
efficiently as possible in terms of memory storage. The efficient use of memory
storage is motivated by the fact that images often require a large numbers of
array locations: an image of 400 by 600 pixels will require 240,000 data points,
each of which will need one or more bytes depending of the data format.
The four different image classes or encoding schemes are: indexed images,
RGB images, intensity images, and binary images. The first two classes are used
to store color images. In indexed images, the pixel values are, themselves, in-
dexes to a table that maps the index value to a color value. While this is an
efficient way to store color images, the data sets do not lend themselves to
arithmetic operations (and, hence, most image processing operations) since the
results do not always produce meaningful images. Indexed images also need an
associated matrix variable that contains the colormap, and this map variable
needs to accompany the image variable in many operations. Colormaps are N
by 3 matrices that function as lookup tables. The indexed data variable points
to a particular row in the map and the three columns associated with that row
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
54.
contain the intensity of the colors red, green, and blue. The values of the three
columns range between 0 and 1 where 0 is the absence of the related color and
1 is the strongest intensity of that color. MATLAB convention suggests that
indexed arrays use variable names beginning in x.. (or simply x) and the sug-
gested name for the colormap is map. While indexed variables are not very
useful in image processing operations, they provide a compact method of storing
color images, and can produce effective displays. They also provide a conve-
nient and flexible method for colorizing grayscale data to produce a pseudocolor
image.
The MATLAB Image Processing Toolbox provides a number of useful
prepackaged colormaps. These colormaps can implemented with any number of
rows, but the default is 64 rows. Hence, if any of these standard colormaps are
used with the default value, the indexed data should be scaled to range between
0 and 64 to prevent saturation. An example of the application of a MATLAB
colormap is given in Example 10.3. An extension of that example demonstrates
methods for colorizing grayscale data using a colormap.
The other method for coding color image is the RGB coding scheme in
which three different, but associated arrays are used to indicate the intensity of
the three color components of the image: red, green, or blue. This coding
scheme produces what is know as a truecolor image. As with the encoding used
in indexed data, the larger the pixel value, the brighter the respective color. In
this coding scheme, each of the color components can be operated on separately.
Obviously, this color coding scheme will use more memory than indexed im-
ages, but this may be unavoidable if extensive processing is to be done on a
color image. By MATLAB convention the variable name RGB, or something
similar, is used for variables of this data class. Note that these variables are
actually three-dimensional arrays having dimensions N by M by 3. While we
have not used such three dimensional arrays thus far, they are fully supported
by MATLAB. These arrays are indexed as RGB(n,m,i) where i = 1,2,3. In fact,
all image variables are conceptualized in MATLAB as three-dimensional arrays,
except that for non-RGB images the third dimension is simply 1.
Grayscale images are stored as intensity class images where the pixel
value represents the brightness or grayscale value of the image at that point.
MATLAB convention suggests variable names beginning with I for variables
in class intensity. If an image is only black or white (not intermediate grays),
then the binary coding scheme can be used where the representative array is a
logical array containing either 0’s or 1’s. MATLAB convention is to use BW for
variable names in the binary class. A common problem working with binary
images is the failure to define the array as logical which would cause the image
variable to be misinterpreted by the display routine. Binary class variables can
be specified as logical (set the logical flag associated with the array) using the
command BW = logical(A), assuming A consists of only zeros and ones. A
logical array can be converted to a standard array using the unary plus operator:
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
55.
A = ؉BW. Since all binary images are of the form “logical,” it is possible to
check if a variable is logical using the routine: isa(I, ’logical’); which will
return a1 if true and zero otherwise.
Data Formats
In an effort to further reduce image storage requirements, MATLAB provides
three different data formats for most of the classes mentioned above. The uint8
and uint16 data formats provide 1 or 2 bytes, respectively, for each array ele-
ment. Binary images do not support the uint16 format. The third data format,
the double format, is the same as used in standard MATLAB operations and,
hence, is the easiest to use. Image arrays that use the double format can be treated
as regular MATLAB matrix variables subject to all the power of MATLAB and
its many functions. The problem is that this format uses 8 bytes for each array
element (i.e., pixel) which can lead to very large data storage requirements.
In all three data formats, a zero corresponds to the lowest intensity value,
i.e., black. For the uint8 and uint16 formats, the brightest intensity value (i.e.,
white, or the brightest color) is taken as the largest possible number for that
coding scheme: for uint8, 28-1
, or 255; and for uint16, 216
, or 65,535. For the
double format, the brightest value corresponds to 1.0.
The isa routine can also be used to test the format of an image. The
routine, isa(I,’type’) will return a 1 if I is encoded in the format type, and
a zero otherwise. The variable type can be: unit8, unit16, or double. There
are a number of other assessments that can be made with the isa routine that
are described in the associated help file.
Multiple images can be grouped together as one variable by adding an-
other dimension to the variable array. Since image arrays are already considered
three-dimensional, the additional images are added to the fourth dimension.
Multi-image variables are termed multiframe variables and each two-dimen-
sional (or three-dimensional) image of a multiframe variable is termed a frame.
Multiframe variables can be generated within MATLAB by incrementing along
the fourth index as shown in Example 10.2, or by concatenating several images
together using the cat function:
IMF = cat(4, I1, I2, I3,...);
The first argument, 4, indicates that the images are to concatenated along
the fourth dimension, and the other arguments are the variable names of the
images. All images in the list must be the same type and size.
Data Conversions
The variety of coding schemes and data formats complicates even the simplest
of operations, but is necessary for efficient memory use. Certain operations
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
56.
require a given data format and/or class. For example, standard MATLAB oper-
ations require the data be in double format, and will not work correctly with
Indexed images. Many MATLAB image processing functions also expect a spe-
cific format and/or coding scheme, and generate an output usually, but not al-
ways, in the same format as the input. Since there are so many combinations of
coding and data type, there are a number of routines for converting between
different types. For converting format types, the most straightforward procedure
is to use the im2xxx routines given below:
I_uint8 = im2uint8(I); % Convert to uint8 format
I_uint16 = im2uint16(I); % Convert to uint16 format
I_double = im2double(I); % Convert to double format
These routines accept any data class as input; however if the class is
indexed, the input argument, I, must be followed by the term indexed. These
routines also handle the necessary rescaling except for indexed images. When
converting indexed images, variable range can be a concern: for example, to
convert an indexed variable to uint8, the variable range must be between 0 and
255.
Converting between different image encoding schemes can sometimes be
done by scaling. To convert a grayscale image in uint8, or uint16 format to an
indexed image, select an appropriate grayscale colormap from the MATLAB’s
established colormaps, then scale the image variable so the values lie within the
range of the colormap; i.e., the data range should lie between 0 and N, where N
is the depth of the colormap (MATLAB’s colormaps have a default depth of
64, but this can be modified). This approach is demonstrated in Example 10.3.
However, an easier solution is simply to use MATLAB’s gray2ind function
listed below. This function, as with all the conversion functions, will scale the
input data appropriately, and in the case of gray2ind will also supply an appro-
priate grayscale colormap (although alternate colormaps of the same depth can
be substituted). The routines that convert to indexed data are:
[x, map] = gray2ind(I, N); % Convert from grayscale to
% indexed
% Convert from truecolor to indexed
[x, map] = rgb2ind(RGB, N or map);
Both these routines accept data in any format, including logical, and pro-
duce an output of type uint8 if the associated map length is less than or equal
to 64, or uint16 if greater that 64. N specifies the colormap depth and must be
less than 65,536. For gray2ind the colormap is gray with a depth of N, or the
default value of 64 if N is omitted. For RGB conversion using rgb2ind, a
colormap of N levels is generated to best match the RGB data. Alternatively, a
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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